In recent years, widely employed as recording materials employed for printers, printing presses, markers and writing means, inking materials, and especially as ink-jet ink, are those comprised of an aqueous solution of water-soluble dyes as a main component, and those comprised of a minute pigment particle dispersion as a main component.
Ink employing water-soluble dyes results in brighter color reproduction, compared to pigment ink. However, when compared to pigment ink, bleeding tends to occur on recording paper sheets due to the fact that the ink is comprised of an aqueous dye solution, and water resistance is also inferior to pigment ink. Further, problems occur in which lightfastness is much lower than pigment ink. On the other hand, many efforts have been made to improve water resistance and lightfastness of pigment ink. However, it is hard to mention that problems of pigment ink have sufficiently been solved.
Contrary to this, water-based ink which is prepared by coloring water-dispersible resins with oil-soluble dyes or hydrophobic dyes to be usable as a water-based dispersion, namely water-based ink employing minute colored polymer particles as a recording material is expected to be one which is used in place of the aforesaid ink employing water-soluble dyes. Heretofore, investigations have been conducted regarding various aspects such as dyes, polymer compositions, a decrease in size of colored microparticles, or constitutions.
For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 54-58504 discloses ink which is prepared by dispersing a mixture consisting of a hydrophobic dye solution and minute vinyl polymer particles in the form of an oil-in-water type; JP-A Nos. 55-139471, 3-250069, and 54-58504 describe ink employing emulsion polymerization or dispersion polymerization particles colored with dyes; JP-A No. 2001-98194 discloses ink comprised of copolymer resins comprised of a hydrophilic polymerization chain portion and a hydrophobic polymerization chain portion; JP-A No. 2000-191968 discloses ink in which vinyl polymer polymerizable unsaturated acid monomers, a hydroxyl group containing monomers, and styrene macromers are employed; JP-A No. 9-1575098 discloses ink in which polyester resins containing cyclohexanedicarboxylic acid are employed; and U.S. Pat. No. 6,384,108 discloses minute particles which are formed by dissolving self-emulsification type polymers in solvents together with dyes and emulsifying the resulting mixture in a water phase.
However, all these minute particles are employed in ink, while mixed with various water-based organic solvents. As a result, when colored microparticles prepared by these methods are employed to prepare ink-jet ink, major problems occur due to insufficient solvent resistance.
On the other hand, in order to overcome the problems due to insufficient solvent resistance, surface crosslinked core/shell particles are disclosed (Patent Document 1). Herein, it is intended that the solvent resistance is enhanced through crosslinking of resins. However, mere surface crosslinking does not result in enhancement of solvent resistance as desired. In addition, problems occur in which, since polymerization is carried out in such a manner that dyes are dissolved in monomers, dyes exhibit a relatively large polymerization inhibiting action and particles tend to coagulate due to crosslinking among the particles when the particle surface is subjected to crosslinking to increase the molecular weight.
Further, polymer particles are described which are provided with a self-emulsification property by an ionic group and are crosslinked via a urethane bond (Patent Document 2). Herein, in the same manner as above, the molecular weight is increased through crosslinking and the solvent resistance of colored microparticles is enhanced. When crosslinking is effectively introduced, the solvent resistance is enhanced due to an increase in the molecular weight of resins. However, no description is made regarding more effective enhancement of the solvent resistance utilizing the core/shell structure.
As noted above, only by introducing crosslinking into the resins of colored microparticles or simply crosslinking the surface of core/shell particles, the resulting enhancement of solvent resistance is not as effective as desired. As a result, colored microparticles which satisfy all performance at a desired balance have not been prepared.